Research On Harmonic Suppression And Resonance Damping Strategy Of MMC-SAPF Based On PSC-PWM

With the development of power electronics technology,there are more and more occasions for medium-high voltage and large capacity,and the problem of power quality is increasingly prominent.More and more nonlinear loads,inject a large number of harmonics into the medium-high voltage systems,causing harmonic pollution.In order to improve the power factor of the system,it is often necessary to use a reactive power compensation capacitor,such as a thyristor switched capacitor.However,the capacitor easily resonates in parallel with the grid-side inductance,amplifying the PCC voltage and grid-side current,and deteriorating the power quality.Because modular multilevel has the advantages of high equivalent switching frequency,high modularity,it is more and more widely used in medium-high voltage and high power occasions.Therefore,this paper studies the problem of harmonic suppression based on MMCSAPF.At the same time,it expands its damping function and applies it innovatively to the management of resonance in the medium voltage distribution network.This article first studies the modulation strategy of MMC-SAPF.With the aid of the dual Fourier analysis of the output voltage characteristics,which modulated by the multi-carrier strategy,the law of the equivalent switching frequency of the output phase voltage under the PSC-PWM and LS-PWM of the corresponding level is obtained.By building an experimental prototype of three-phase twelve sub-modules,high-frequency rectification experiments were carried out.The spectrum analysis of the bridge arm voltage,equivalent output phase voltage,bridge arm current,and device output current waveforms verifies the characteristics of PSC-PWM modulation from the experimental point of view,and also explains the reason for choosing this modulation strategy in this article.Aiming at the harmonic problem of medium voltage distribution network,this paper uses MMC-SAPF to suppress harmonics.In order to ensure the balance of the sub-module capacitor voltage,this paper proposes a control strategy from three levels.In order to extract the harmonic commands more accurately and simplify the extraction operation,this paper adopts the specified sub-harmonic extraction strategy based on the RDFT algorithm.Aiming at the harmonic suppression scenario,a composite control strategy using PI parallel RC in the current loop is proposed.Finally,this paper proposes a specified sub-harmonic suppression strategy based on MMC-SAPF.The experiment was carried out by building an experimental prototype and connecting an uncontrolled rectifier load.Both steady-state and dynamic test results verify that MMC-SAPF has the ability to suppress harmonics.Since the resonance problem is usually related to the harmonic problem,this article provides an innovative solution to the resonance problem in the medium voltage distribution network caused by the reactive power compensation capacitor.That is,on the basis of MMC-SAPF harmonic suppression,its damping function is expanded,and a comprehensive control strategy of harmonic suppression and resonance damping is proposed.Aiming at the problem that the resonant frequency may change,an adaptive gain resonant damping command extraction strategy is designed.By building an experimental prototype,a 7-order resonance environment was constructed for experimentation.Both steady-state and dynamic tests have verified the ability of MMCSAPF to have resonance damping.